Press-on forces play a major role in dry powder inhaler (DPI) formulations. In this work, we have quantified the press-on forces on different substrates and drugs under controlled conditions. Externally applied forces significantly affect drug adhesion to surfaces and were found to be independent of the type of substrate and drug. A conservative estimate of press-on forces was made and was found to depend inherently on the size and shape of the drug particle and also on the contact area of the drug and substrate. These press-on forces were positively correlated to the mixing energy. It was found to increase quadratically with an increase in mixing speed. This work is of relevance to the pharmaceutical process of mixing DPI formulations, as it relates dispersion performance to the mixing time, batch size and carrier and drug properties. (C) Koninklijke Brill NV, Leiden, 2011